It's a reinterpretation of oral accounts passed down decades after the deaths of the people it's about, and was first attributed to John nearly 180 years later. The gospel of John was first authored anonymously around 90-100ad and attributed to John by Irenaeus in 185ad

90-100ad isn't decades after the death of people it's about.

And it wasn't authored anonymously.

John 21:20-25 ESV

[20] Peter turned and saw the disciple whom Jesus loved following them, the one who also had leaned back against him during the supper and had said, “Lord, who is it that is going to betray you?” [21] When Peter saw him, he said to Jesus, “Lord, what about this man?” [22] Jesus said to him, “If it is my will that he remain until I come, what is that to you? You follow me!” [23] So the saying spread abroad among the brothers that this disciple was not to die; yet Jesus did not say to him that he was not to die, but, “If it is my will that he remain until I come, what is that to you?” [24] This is the disciple who is bearing witness about these things, and who has written these things, and we know that his testimony is true. [25] Now there are also many other things that Jesus did. Were every one of them to be written, I suppose that the world itself could not contain the books that would be written.

That this disciple reclined with Jesus and was at the crucifixion and resurrection.

John 13:23 ESV

One of his disciples, whom Jesus loved, was reclining at table at Jesus’ side,

John 19:25-26 ESV

but standing by the cross of Jesus were his mother and his mother’s sister, Mary the wife of Clopas, and Mary Magdalene. [26] When Jesus saw his mother and the disciple whom he loved standing nearby, he said to his mother, “Woman, behold, your son!”

John 20:2-5

[2] So she ran and went to Simon Peter and the other disciple, the one whom Jesus loved, and said to them, “They have taken the Lord out of the tomb, and we do not know where they have laid him.” [3] So Peter went out with the other disciple, and they were going toward the tomb. [4] Both of them were running together, but the other disciple outran Peter and reached the tomb first. [5] And stooping to look in, he saw the linen cloths lying there, but he did not go in.

So this is clearly a primary source.

More reasoning for narrowing it down to John can be found here.

It also makes sense that somebody would know who wrote the Gospel. The authorship of the Gospels were never disputed in the early Church despite geographic spread. So that doesn't mean that Irenaeus (A student of Polycarp who was a disciple of John) made it up

Not really, contemporary sources are generally limited to people involved with the actual history.

Which the writer of John clearly was.

When combined with other contextual sources.

There are four detailed accounts of Jesus.

I'm not the one making the claim that other religions are wrong and Christianity is true. Do you have evidence that Joseph Smith, Muhammad, or Buddha had alternative motives?

Mormonism has been debunked by the finding of the "original" papyrus to one of their scriptures. The Qur'an claims to be in agreement with the Bible yet contradicts the hell out of it. Mohammed and JS had numerous wives because "god told me" and Mohammed was a warlord, JS tried to set up "deseret"